Conventional hot water heaters, especially those with electric resistance heating, are being converted so that the water in the water heater is being heated by heat pumps externally of the water heater rather than with electric resistance heating. One of the problems in converting such hot water heaters is the connection of the heat pump to the water heater and the installation of a thermostatically operated controller on the water heater to control the operation of the heat pump. Typically, the heat pump is connected to the hot water heater through available openings in the hot water heater for the withdrawal of the water from the hot water heater to be heated in the heat pump and for return of the heated water to the water heater. The thermostatic controller is typically connected to the outside of the water tank in the heater and under the insulation thereon so as to be responsive to the temperature of the water in the heater water tank. Because the outside of the water heater is typically covered with a metal shell, the location of the thermostatic controller on the outside of the tank is usually limited to those portions of the outside of the tank accessible through this outer shell. On the other hand, the thermostatic controller needs to be located so that it is responsive to the water temperature at some particular height in the tank to achieve reasonable cycles of operation of the heat pump while at the same time providing a reasonable hot water reserve in the heater water tank. This has created problems in the installation of the thermostatic controller since the locating of the controller is dependent on the capability of the installer and since the accessible portions of the outside of the hot water tank may not be located so that the thermostatic controller installed at the accessible portion of the heater water tank will be responsive to the water at the desired height in the tank. Further, installation of the thermostatic controller on the outside of the heater water tank is sometimes precluded by applicable government codes. As a result, the connection of the heat pump to the water heater has been a time consuming operation.